It will only "auto" download the multi GB file if you are on someone else's wi-fi AND you have hard power connected (USB, adapter plug etc).
It will not auto-download if you are just walking around with the device even if on wi-fi. Now - you may get a notification of it being available when walking around on wi-fi but the file will not actually come down via wi-fi alone.
Most folks get caught by this when charging their device. If you do not put your device in Airplane mode - or better yet - plug in and immediately power it down - you will eventually get caught with a huge file on your device at some point. As soon as that hard power is connected and wi-fi is still enabled - it will try relentlessly to get the file.
I tested this extensively when blocking the iOS7 OTA update last year. I have no reason to suspect otherwise with iOS8 in the queue now.
Sonic